The interest in two dimensional (2D) materials results from the fact that optoelectronics and molecular electronics have become frontier areas of material science (Ulman, 1991). Multilayered organic structures have recently received theoretical (Lam, et al., 1991) and experimental (So, et al., 1991; Forrest, et al., 1994; Haskal, et al., 1994; Ohmori, et al., 1993; Yoshimura, et al., 1991; Yoshimura, et al., 1992; Donovan, et al., 1993; Donovan, et al., 1994) treatment. Novel and applicable photophysical properties of organic superlattices have been predicted, including enhancement of optical nonlinearities (Lam, et al., 1991; Zakhidov and Yoshino, 1994) and photoelectric transformations (Zakhidov and Yoshino, 1994). Techniques such as organic molecular beam deposition (OMBD) (So, et al., 1991; Forrest, et al., 1994; Haskal, et al., 1994; Ohmori, et al., 1993) have already proved the capability of growing ultrathin layers having the quality of inorganic quantum well (QW) structures.
A number of interesting optical and photophysical phenomena have already been found in OMBD derived organic QW, including the observation of exciton confinement in photoluminescence (PL) (So, et al., 1991; Forrest, et al., 1994) and electroluminescence (EL) and electric field modulation of PL (Ohmori, et al., 1993). Preparation of crystalline thin organic films by the OMBD relies on the bonding of molecular layers via weak van der Waals forces to achieve and preserve quasi-epitaxial structures (Forrest, et al., 1994). Thus, perfect monolayers without step edges cannot be achieved and the lower limit is an average of three "monomolecular" layers. A solution to these limitations can be found in another ultrahigh vacuum (UHV) technique: molecular layer deposition (MLD) (Yoshimura, et al., 1991; Yoshimura, et al., 1992). MLD demonstrated the construction of quantum dots and quantum wires and the potential use of functionalized organic precursors to form alternating multilayered structures. This approach is similar to (inorganic) atomic layer deposition (ALD) (Pessa, et al., 1981) and the solution analog-molecular self-assembly (MSA) (Ulman, 1991). MLD is now at a stage that challenges chemists to introduce new and efficient optoelectronically-active molecular precursors to such a process.
A solution derived method to produce 2D layered structures, the Langmuir-Blodgett (LB) technique, yields films exhibiting anisotropic electron transport (Donovan, et al., 1994) and tunneling (Donovan, et al., 1993), again suggesting QW behavior. While the LB method is useful in achieving 2D multilayered physiadsorbed structures, LB films suffer from low chemical and thermal stability and cannot incorporate large chromophores without phase-segregation and micro-crystal formation. Alternatively, the trichlorosilane-based MSA approach 1 provides the advantages of strong chemiadsorption through Si--O bonds, chemical and thermal stability, and the ability to form noncentro symmetric structures (Yitzchaik, et al., 1993; Li, et al., 1990).
Vapor phase growth techniques (Kubono, et al., 1994) such as vapor deposition polymerization (VDP) of thin films was recently demonstrated for aromatic polymers such as polyimides (Maruo, et al., 1993), polyamides (Takahashi, et al., 1991), polyureas (Wang, et al., 1933), and polyether-amines (Tatsuura, et al., 1992). In the VDP process, two types of monomers are evaporated onto the surface of a substrate in a vacuum chamber. Condensation or addition polymerization then takes place between deposited monomers to produce thin polymeric films. Thin polymer films of high quality and uniformity can be fabricated by this process (Maruo, et al., 1993; Takahashi, et al., 1991). Thermally stable piezo- and pyro-electrical properties were found in poled samples (Wang, et al., 1993). Moreover, electric field assisted VDP (in situ poling of hyperpolarizable monomers) was employed to fabricate electro-optic polymer waveguides (Tatsuura, et al., 1992).